I don't much about SMPS but I understand that the mains is directly rectified at the input. This is a practice that is against forum rules so we can't discuss the design of such supplies. However, we can go and buy them and stick them in a project chasis. The efficiency of SMPS makes their use of great interest because of it's environmental benefit and so I believe a number of hobbyists consider using them; this is something to be encouraged. They are also a lot lighter when you consider the weight of a large transformer for a high power amplifier - a safety factor of it's own perhaps.

I want to know what kind of safety precautions are important for using these kinds of supplies - hopefully this is something that is allowed as a topic. Is it just simply a case of using one with an isolating output transformer (as many are designed this way) along with adequate ground connection between chasis and safety earth ? are there other desirable safety features, such as buying supplies that use plastic encapsulated power devices (less risk of contact with metal heatsink) or other such things to look for ?

__________________"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Look for supplies with adequate physical separation between live and DC sides. I have heard of some Chinese designs which expect a mica thermal washer under a power transistor to withstand mains voltage. There are safety rules about this sort of thing (e.g. Low Voltage Directive in Europe) but some suppliers ignore them because they know enforcement is patchy.